The effect of methylation of lac gene DNA and of other factors required on its coded enzyme beta-galactosidase synthesis is being studied to determine how akylation affects fidelity of gene function. Increasing concentrations of dimethyl sulfate (DMS) up to 4mM methylating the DNA at 37 degrees C for 10 minutes, caused a linear inhibition on this cell-free enzyme synthesis to about 30-40% of the control DNA synthesized beta-galactosidase level. Total protein and RNA synthesis were found to be much less inhibited. Besides DNA, S-30 (the 30,000 xg E. coli cellular extract) methylation was found to contribute greatly on the inhibition of normal beta-galactosidase synthesis. The newly formed galactosidases from DMS methylated DNA appeared to be the same as that from control-treated DNA by the criteria of heat inactivation at 57 degrees C and also the various substrate concentration studies. N-nitrosomethylurea (MNU) also caused DNA to synthesize a lesser amount of active beta-galactosidase, but this strong carcinogen differs from DMS in the site of DNA base methylation. MNU's effect on S-30 is also different from that of DMS. The possible mechanisms of both carcinogens on this system are explored.